Final answer:
The average paO2 on the venous side of a pulmonary capillary bed is around 100 mm Hg when blood leaves the lungs and enters the systemic capillaries. This decreases to around 40 mm Hg as the blood moves through the capillaries and into the tissue cells.
Step-by-step explanation:
When blood leaves the lungs through the pulmonary veins and enters the systemic capillaries, the average paO2 (partial pressure of oxygen) on the venous side of the pulmonary capillary bed is typically around 100 mm Hg. This means that the blood in the pulmonary capillaries is highly oxygenated. As the blood moves through the capillaries and into the tissue cells, the paO2 decreases to around 40 mm Hg due to oxygen diffusion into the tissues.